Molecular sieve crystals are generally microporous structures composed of either crystalline aluminosilicate, belonging to a class of materials known as zeolites, or crystalline aluminophosphates, or crystalline metalloaluminophosphates such as silicoaluminophosphates. The crystals are conventionally made by hydrothermal crystallization from a reaction mixture comprising reactive sources of silicon and/or aluminum and/or phosphorous containing compounds, usually in the presence of one or several organic amine or quaternary ammonium salts.
Molecular sieve catalysts are compositions made of molecular sieve crystal particles bound together to form a formulated catalyst material. The formulated molecular sieve catalyst typically includes other components such as binders, fillers such as clay, and, optionally, other catalytically active agents such as rare earth metal oxides, transition metal oxides, or noble metal components.
Conventional methods of making molecular sieve catalysts include mixing together molecular sieve and binder, as well as other optional components such as fillers and other catalytic components. The mixture is typically stirred in solution to form a slurry, and the slurry is dried to form molecular sieve catalyst particles. Following drying, the particles are calcined to remove templates present in the molecular sieve, to harden, as well as to activate the catalyst.
U.S. Pat. No. 4,764,269 (Edwards) discloses conventional methods of making and using SAPO-37 molecular sieve catalyst that can be used in catalytic cracking operations. The catalyst was found to be adversely affected by moisture, but the crystalline structure and activity of the molecular sieve component was preserved by including a stabilizing amount of the organic template compound used in the manufacture of the molecular sieve within the pore structure thereof until such time as the catalyst was thermally activated during use.
Metalloaluminophosphate molecular sieves, such as the SAPO-37 molecular sieve described by Edwards, have a variety of uses. A desirable characteristic for many of the metalloaluminophosphate molecular sieves, regardless of the process of use, is that the finished or formulated catalyst be attrition resistant, which can refer to hardness as well as ability to absorb shock, since the catalyst will typically have to endure severe stress in commercial scale processes.
For example, WO 99/21651 describes a method for making molecular sieve catalyst that is considered relatively hard. The method includes the steps of mixing together a molecular sieve and an alumina sol, the alumina sol being made in solution and maintained at a pH of 2 to 10. The mixture is then spray dried and calcined. The calcined product is reported to be relatively hard, i.e., attrition resistant.
U.S. Pat. No. 6,334,994, incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, referred to as RUW-19, which is said to be an AEI/CHA mixed phase composition. In particular, RUW-19 is reported as having peaks characteristic of both CHA and AEI framework type molecular sieves, except that the broad feature centered at about 16.9 (2θ) in RUW-19 replaces the pair of reflections centered at about 17.0 (2θ) in AEI materials and RUW-19 does not have the reflections associated with CHA materials centered at 2θ values of 17.8 and 24.8. DIFFaX analysis of the X-ray diffraction pattern of RUW-19 as produced in Examples 1, 2 and 3 of U.S. Pat. No. 6,334,994 indicates that these materials are characterized by single intergrown phases of AEI and CHA framework type molecular sieves with AEI/CHA ratios of about 60/40, 65/35 and 70/30, respectively. Throughout this description, the XRD reflection values are, referred to as (2θ), which is synonymous to the expression “degrees 2θ.”
U.S. Pat. No. 6,812,372, incorporated herein by reference, discloses a silicoaluminophosphate molecular sieve, comprising at least one intergrown phase of molecular sieves having AEI and CHA framework types, wherein the intergrown phase has an AEI/CHA ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of the silicoaluminophosphate molecular sieve.
U.S. Pat. No. 6,953,767 and US Patent Publication No. 2005-0096214 A1 disclose a silicoaluminophosphate molecular sieve comprising at least one intergrown phase of molecular sieves having AEI and CHA framework types, wherein the intergrown phase has an AEI/CHA mass ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of the silicoaluminophosphate molecular sieve. It also relates to methods for its preparation and to its use in the catalytic conversion of methanol to olefins.
U.S. Pat. No. 6,153,552 describes another method for making molecular sieve catalyst. The catalyst is made by mixing together a silicon containing oxide sol as a binder material and a molecular sieve material. The pH of the mixture is adjusted prior to spray drying. Following spray drying, the catalyst material is calcined to form a finished catalyst product, which is reported to be relatively hard, i.e., attrition resistant.
A suitable attrition index continues to be a desirable characteristic in molecular sieve catalysts. As new process systems are developed, the ability to control a desired attrition index of a catalyst is particularly important so as to improve the catalytic process and process economics. A lower attrition index translates into lower catalyst loss rate, therefore, lower catalyst make-up rate and better process economics. Therefore, obtaining molecular sieve catalysts that have a desired degree of attrition index are still sought.